According to Tim McCreight, metal can be heat hardened by holding it
at annealing temperature. I put fine silver in a tabletop kiln (1200
degrees F) for two hours but it stayed soft.
Another author says to harden silver in a 600 degree F oven for a
half hour. My oven only goes up to 550 degrees (although it may
reach 600 during the cleaning cycle), so I haven’t tested this.
How do you heat harden fine silver? Can it be softened again
afterwards?
Janet
How do you heat harden fine silver? Can it be softened again afterwards?
Janet, you cannot heat harden fine silver. The process you refer to
is called age hardening, or precipitation hardening, and applies to
silver alloyed with copper, such as sterling silver. It works
because at elevated temperatures that are still below the annealing
temperature, the copper forms separate crystals at the grain
boundaries between the normal silver crystals. And yes, you can
again anneal this type of hardened silver by heating it hotter, to
the normal annealing temperatures, which forces the copper back into
solution again. Since fine silver has no copper, this cannot happen,
so no real hardening. The only real way to harden fine silver is to
work harden it. forging, bending, burnishing, etc.
Peter
I tried to heat harden a bracelet in my oven using the cleaning
cycle- I had to clean the oven anyway- and it seemed to work pretty
well.
Janet Kofoed
Hi Janet, Try 350 F for one hour. You can use you mold vulcanizer to
do that if you don’t have a controlled oven. Afterwards, you can
anneal it again to make it soft. Good luck. Brian
the copper forms separate crystals at the grain boundaries annealing...forces the copper back into solution again.
Peter, Fascinating–thank you for explaining precipitation hardening.
Does it matter if metal is quenched after annealing?
Janet
age hardening or heat hardening can only occur with an alloy, and
for us that is an alloy containing copper like sterling silver or
most gold alloys. The heating time (which can vary but works at 450
degrees F or more) can be as little as a hour and go much longer. The
heat makes the crystals (grains) of the metal enlarge and migrates
brittel (copper oxides) towards the grain boundaries thus producing a
three dimensional ‘lace’ of stiffer materials at the outsides of the
grains. Thus the whole piece gets harder. I suspect it might
contribute to fire scale (don’t know), and the stiffness you get
varies with the alloy, up to a 200% increase over dead soft in some
metals. But a solid burnishing hardens just as well. It is useful for
production situations, and I met a goldsmith who had forged earrings
where the earring wire was forged out of the mair part of the
earring. In his case age hardening stopped the wires from breaking
by making them stiffer. You can think of metal grains as looking just
like grapes that have been stuffed in a crate, rounded shapes with
flattened geometric sides where they abut other crystals (this is
also the same shape as human fat cells). best Charles
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The heat makes the crystals (grains) of the metal enlarge and migrates brittel (copper oxides) towards the grain boundaries thus producing a three dimensional 'lace' of stiffer materials at the outsides of the grains. Thus the whole piece gets harder. I suspect it might contribute to fire scale (don't know)
Close, Charles. Not quite, though.
The heat does indeed cause grain growth, though at these lower
temperatures, usually that’s not all that much. Precipitation
hardening works because the metallic copper is not actually properly
soluble in the gold or silver at lower temperatures, and at these
temperatures it’s mobile enough that it can come out of the forced
solution (forced there just because it’s soluble in the molten metal
and at high temperatures, but doesn’t have time to come out of
solution in normal cooling of a casting piece of metal or working
it. Annealing, for example heats the metal high enough to again
dissolve the copper in the gold) the hardening that results from
precipitation hardening is because the copper, coming out of
solution, forms new crystals of copper along the grain boundaries
between the gold crystals.
Metals deform with two mechanisms. One is that due to the atoms
literally sliding along the slip planes within the crystals
themselves, the crystals can be very much deformed. But for a whole
mass of metal to deform, not only must the crystals deform, but the
boundaries between the crystals must also stretch and deform to
follow the deformation of the crystals themselves
The boundary between like crystals is far more able to deform than a
boundary between unlike crystals, so the formation of different
crystals, in this case copper, along crystal boundaries, then means
that even if the crystals themselves are soft (copper is, of course,
soft), the boundaries between the crystals cannot so easily stretch
and deform to follow the deformation of the crystals themselves. This
then keeps the whole mass of metal from deforming.
There shouldn’t be copper oxides within the body of the metal.
That’s not what’s migrating around in precipitation hardening, at
least not what the intent is. Regarding fire scale, If you harden the
metal in an “air” environment, oxygen will penetrate into the surface
of the metal, and if it meets copper, will combine with it. The
resulting copper oxide tends to migrate to the surface if it’s near
to it in the first place, (at elevated temperatures, any of the
metal ions tend to migrate around, the mechanism of diffusion
bonding, for example) If it reaches the surface, it stays there,
since the oxide isn’t as able to rediffuse into the alloy, so it
builds up on the surface. . So yes, precipitation hardening can
cause fire scale, but the mechanism of that is slightly different,
and need not happen if the surface of the metal being hardened is
protected from oxygen.
Peter
Peter, Fascinating--thank you for explaining precipitation hardening. Does it matter if metal is quenched after annealing?
Yes. Some metals can start to age harden quite quickly, and
quenching prevents that from happening as they slow cool. gold
alloys, especially white golds, do this more than yellow golds, or
silver, but even sterling silver is a little softer if quenched than
if allowed to slow cool. You do have to be careful not to quench
from too high a temperature, though, as this can sometimes crack the
metal. Usually, quench just as the last trace of red glow is gone,
and you’ll be fine.
When age hardening, though, quenching after you’ve heat treated the
material to harden it has no effect other than to get you done more
quickly. It’s not like steels, where the quenching operation is
critical to actually getting the hardness.
Precipitation or age hardening, by the way, is most effective after
an initial anneal in which you actually heat the metal rather higher
than normal annealing suggests. What that does is allow considerable
grain growth. Metal with larger but fewer individual crystals then
has less total surface area of crystal boundaries, so that after
precipitation hardening, the concentration of the copper crystals
along those boundaries is higher than it would be if the grain
structure were smaller. And that means those fewer grain boundaries
are more resistant to deformation, so the metal is then a bit harder.
Peter